The objective of this study is to gain more information about the morphological phenotype associated with trisomy 19 in the mouse during the second half of gestation. The methods for a majority of the study will be those of classical embryology, and will include a careful examination of the progression of external phenotypic development from 10 through 17 gestational days, and examination of serially sectioned animals and their placentas during the same period. The information which is gained from these studies will then be cmpared with descriptive data obtained from the normal littermates of the trisomic animals. The trisomic animals are growth-retarded, and this comparison of the trisomic and normal animals will provide information concerning the effect of this retardation on morphogenesis. If any resulting observations suggest that altered cell proliferative activity is a probable cause for any observed retardation, labeled mitoses studies will be carried out with that structure or system in vivo, using the technique of tritiated thymidine autoradiography.